This application claims the priority of Korean Patent Application No. 2003-25996, filed on Apr. 24, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to an apparatus for manufacturing a billet for thixocasting, and more particularly, to an apparatus for manufacturing a billet for thixocasting with a fine and uniform particle structure
2. Description of the Related Art
Thixocasting is closely related to rheocasting and thus is also expressed as rheocasting/thixocasting. Rheocasting refers to a process of manufacturing billets or final products from semi-solid metallic slurries with a predetermined viscosity, through casting or forging. Thixocasting refers to a process involving reheating billets, manufactured through rheocasting, back into semi-molten slurries and casting or forging the slurries to obtain final products. Semi-solid metallic slurries consist of spherical solid particles suspended in a liquid phase in an appropriate ratio at temperature ranges of a semi-solid state. Thus, they can be transformed even by a little force due to their thixotropic properties and can be easily cast like a liquid due to their high fluidity.
Such rheocasting/thixocasting is more advantageous than general forming processes using molten metals, such as casting or forging. Because semi-solid or semi-molten metallic slurries used in rheocasting or thixocasting have fluidity at a lower temperature than molten metals, it is possible to lower the die casting temperature, thereby ensuring an extended lifespan of the die. In addition, when semi-solid or semi-molten metallic slurries are extruded through a cylinder, turbulence is less likely to occur, and thus less air is incorporated during casting. Therefore, the formation of air pockets in final products is prevented. Besides, the use of semi-solid or semi-molten metallic slurries leads to reduced shrinkage during solidification, improved working efficiency, mechanical properties, and anti-corrosion, and lightweight products. Therefore, such semi-solid or semi-molten metallic slurries can be used as new materials in the fields of automobiles, airplanes, and electrical, electronic information communications equipment.
As described above, billets manufactured by rheocasting are used in thixocasting. In conventional rheocasting, molten metals are stirred at a temperature lower than the liquidus temperature for cooling, to break up dendritic structures into spherical particles suitable for rheocasting, for example, by mechanical stirring, electromagnetic stirring, gas bubbling, low-frequency, high-frequency, or electromagnetic wave vibration, electrical shock agitation, etc.
By way of example, U.S. Pat. No. 3,948,650 discloses a method and apparatus for manufacturing a liquid-solid mixture. In this method, molten metals are vigorously stirred while cooled for solidification. A semi-solid metallic slurry manufacturing apparatus disclosed in this patent uses a stirrer to induce flow of the solid-liquid mixture having a predetermined viscosity to break up dendritic crystalline structures or disperse broken dendritic crystalline structures in the liquid-solid mixture. In this method, dendritic crystalline structures formed during cooling are broken up and used as nuclei for spherical particles. However, due to generation of latent heat of solidification at the early stage of cooling, the method causes problems of low cooling rate, manufacturing time increase, uneven temperature distribution in a mixing vessel, and non-uniform crystalline structure. Mechanical stirring applied in the semi-solid metallic slurry manufacturing apparatus inherently leads to non-uniform temperature distribution in the mixing vessel. In addition, because the apparatus is operated in a chamber, it is difficult to continuously perform a subsequent process.
U.S. Pat. No. 4,465,118 discloses a method and apparatus for manufacturing semi-solid alloy slurries. This apparatus includes a coiled electromagnetic field application unit, a cooling manifold, and a die, which are sequentially formed inward, wherein molten metals are continuously loaded down into the vessel, and cooling water flows through the cooling manifold to cool the outer wall of the die. In manufacturing semi-solid alloy slurries, molten metals are injected through a top opening of the die and cooled by the cooling manifold, thereby resulting in a solidification zone within the die. When a magnetic field is applied by the electromagnetic field application unit, cooling is allowed to break up dendritic crystalline structures formed in the solidification zone. Finally, ingots are formed from the slurries and then pulled through the lower end of the apparatus. The basic technical idea of this method and apparatus is to break up dendritic crystalline structures after solidification by applying vibration. However, many problems arise with this method, such as complicated processing and non-uniform particle structure. In the manufacturing apparatus, since molten metals are continuously supplied to form ingots, it is difficult to control the states of the metal ingots and the overall process. Moreover, prior to applying an electromagnetic field, the die is cooled using water, so that a great temperature difference exists between the peripheral and core regions of the die.
Other types of rheocasting or thixocasting known in the art are described later. However, all of the methods are based on the technical idea of breaking up dendritic crystalline structures after formation, to generate nuclei of spherical particles. Therefore, problems arise, such as those described in conjunction with the above patents.
U.S. Pat. No. 4,694,881 discloses a method for manufacturing thixotropic materials. In this method, an alloy is heated to a temperature at which all metallic components of the alloy are present in a liquid phase, and the resulting molten metals are cooled to a temperature between their liquidus and solidus temperatures. Then, the molten metals are subjected to a shearing force in an amount sufficient to break up dendritic structures formed during the cooling of the molten metals to thereby manufacture the thixotropic materials.
Japanese Patent Application Laid-open Publication No. Hei. 11-33692 discloses a method of manufacturing metallic slurries for rheocasting. In this method, molten metals are supplied into a vessel at a temperature near their liquidus temperature or 50° C. above their liquidus temperature. Next, when at least a portion of the molten metals reaches a temperature lower than the liquidus temperature, i.e., at least a portion of the molten metals begins cooling below their liquidus temperature, the molten metals are subjected to a force, for example, ultrasonic vibration. Finally, the molten metals are slowly cooled into metallic slurries containing spherical particles. This method also uses a physical force, such as ultrasonic vibration, to break up the dendrites grown at the early stage of solidification. In this regard, if the casting temperature is greater than the liquidus temperature, it is difficult to form spherical particle structures and to rapidly cool the molten metals. Furthermore, this method leads to non-uniform surface and core structures.
Japanese Patent Application Laid-open Publication No. Hei. 10-128516 discloses a casting method of thixotropic metals. This method involves loading molten metals into a vessel and vibrating the molten metals using a vibrating bar dipped in the molten metals to directly transfer its vibrating force to the molten metals. After forming a semi-solid and semi-liquid molten alloy, which contains nuclei, at a temperature range lower than its liquidus temperature, the molten alloy is cooled to a temperature at which it has a predetermined liquid fraction and then left stand from 30 seconds to 60 minutes to allow the nuclei to grow, thereby resulting in thixotropic metals. However, this method provides relatively large particles of about 100 μm and takes a considerably long processing time, and cannot be performed in a vessel larger than a predetermined size.
U.S. Pat. No. 6,432,160 discloses a method for making thixotropic metal slurries. This method involves simultaneously controlling the cooling and the stirring of molten metals to form the thixotropic metal slurries. In detail, after loading molten metals into a mixing vessel, a stator assembly positioned around the mixing vessel is operated to generate a magnetomotive force sufficient to rapidly stir the molten metals in the vessel. Next, the molten metals is rapidly cooled by means of a thermal jacket, equipped around the mixing vessel, for precise temperature control of the mixing vessel and the molten metals. During cooling, the molten metals are continuously stirred in a manner such that when the solid fraction of the molten metals is low, a high stirring rate is provided, and when the solid fraction increases, a greater magnetomotive force is applied.
Most of the aforementioned conventional rheocasting and thixocasting methods and apparatuses use shear force to break dendritic structures into spherical structures during a cooling process. Since a force such as vibration is applied after at least a portion of the molten metals is cooled below their liquidus temperature, latent heat is generated due to the formation of initial solidification layers. As a result, there are many disadvantages such as reduced cooling rate and increased manufacturing time. In addition, due to a non-uniform temperature between the inner wall and the center of the vessel, it is difficult to form fine, uniform spherical metal particles. Therefore, this structural non-uniformity of metal particles will be greater if the temperature of the molten metals loaded into the vessel is not controlled.
In order to solve these problems, the present inventor filed Korean Patent Application No. 2003-13516, titled “Method and apparatus for manufacturing billet for thixocasting”.